Adjustment of speed-load characteristics of mill motors



April 4, 1939. E. N.'MILL'.AN ET AL ADJUSTMENT OF SPEED-LOAD CHARACTERISTICS OF MILL MOTORS Filed June 6, 1956 2 Shee ts-Sheet l MUPK b m H w k. Q SQ N w m m N Q w m Q Q Q Q up w 1 P l I P INVENTORS. Eawnea N MILL/1N lva Y P 1 E. N. MILLAN ET AL 2,153,429

ADJUSTMENT OF SPEED-LOAD CHARACTERISTICS OF MILL MOTORS Filed June 6, 1936 2 Sheets-Sheet 2 2/7 v INVENTORSS C [aw/ma NM/LL/I/V Alva BY ERNEST 6. 50/11. UR

ATTORNEYS Patented Apr. 4, i939 u fim s'rATEs ADJUSTMENT 0F SPEED-LOAD CHARACTER ISTICS OF MILL MOTORS Edward N. Millan and Ernest G. Schlup, Middletown, Ohio, aasignors to The American Rolling Mill Company, Middietown, Ohio, a corporation of Ohio Application June s, 1936, Serial No. 83,974

- 6 Claims. (ciao-s5) The invention relates to a device for changing, and predeterminately adjusting the speed curve characteristic of a single hot or cold rolling mill drive-motor, or of several motors driving hot or cold rolling tandem mills respectively. In the desired rate of acceleration or deceleration of the moving parts of the mill drive or drives. It is still another object of-the invention to keep the strip under practically constant tension between the various rolling mill stands.

These and other objects of the invention which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, we accomplish by that certain construction and arrangement oi. parts of which we shall now describe the aforesaid exemplary embodiments, it being understood that modifications thereof may be made without departing from the spirit of the invention, and that the invention may be usefully employed in environments other than the one specifically described herein.

Reference is now made to the drawings, wherein:

Figure 1 is a speed-load graph showing a prac- V tically flat characteristic curve.

Fig. 2 is a similar graph showing a slightly drooping characteristic curve.

Fig. 3 is a similar graph showing a more decidedly drooping characteristic curve.

Fig. 4 shows a typical arrangement of a three stand tandem mill with coiler and uncoiler accessories.

Fig. 5 is a circuit diagram of an arrangement embodying a motor driven series exciter with accessories.

Fig. 6 is a circuit diagram of an arrangement embodying a motor driven booster or bucking generator with accessories.

In order that the invention may be more clearly understood, certain of the general dlsiderata of a tandem rolling mill will now be considered. It is desirable and practical to operate cold or hot tandem mills at low threading speeds, at high rolling speeds, and again at low speeds at the end of a rolling operation. It is desirable and necessary to keep a constant tension in thestrip over the whole rolling operation, including an accelerating, a high speed, and a decelerating rolling cycle.

It is further desirable that the tension to be maintained may be predetermined and pre-set in accordance with varying conditions. It is a matter of general knowledge in the art that each mill in a tandem arrangement, as shown in Fig. 4, is run faster than the mill immediately preceding it, so as to compensate for normal elongation in the strip. Then it is clear thatwith each of the mill motors operating at the proper adjusted speed to make a given reduction in each stand I the delivery of the strip from one mill to the other will be at a constant rate so long as the reduction in each pass remains constant. But sporadic changes may occur to upset such constant delivery speeds so that when the delivery speed is reduced, the tension becomes too high, causing breakage, or when the delivery speed is increased the strip will become too slack. But when a succeeding mill motor has a speed-load curve of a more flat character than that of the motor of the preceding mill, these sporadic changes can be automatically compensated for in the following manner: when No. i mill delivers strip to No. 2 mill at a greater rate than No. 2 mill speed is adjusted to receive, the strip tension between No. l and No. 2 is reduced, thereby increasing the power required on No. I mill motor which will cause No. l mill motor to decrease in speed until the strip is again under tension betweenmills I and 2 and No. l mill motor has returned to the same speed-load conditions at which it was operating beforethe upset occurred.

.From this arrangement the inherent characteristics of the various mill motors of a continuous mill train can be adjusted to have a speed-load curve that will maintain tension between the mills by virtue of the drooping of the speed-load curve above, and without the use of other means such as automatically adjusting the mill motor rheostat.

Referring to Fig. 4, the motor pinions 4, 5 and 6 of the motors I, 2 and 3, respectively, are all of the same size. Gear |,-which engages with the motor pinion l is larger than gear 8 which engages with the motor pinion 5. Gear 8 is itself larger than the gear 9 which engages with the motor pinion 6. The rolls H), II and i2 are of equal size, so that at equal speeds of the motors l, 2 and 3, the roll ll of mill No. 2 revolves faster than the roll ll of mill No. I; and the roll l2 of mill No. I revolves faster than the roll of mill No. 2. It will be apparent to one skilled in the art, upon reading these specifications, that the sum of the polar moments of inertia of the motor-pinion l and -the gear I of mill No. is higher than the sum of the polar moments of inertia of the motor pinion 5 and the gear 8 of mill No. 2. Likewise it will be apparent that the sum of the polar moments of inertia of the motor pinion I and the gear 8 of mill No. 2 is higher than the sum of the polar moments of inertia oi the motor pinion 6 and the gear 9 of mill No. 2. The torque requirements of motors 2 and 3, may be determined from the following formula:

WR'XR. P. as.

Torque 308 X t Consideration has been given above to an arrangement in which the speeds of the motors I, 2 and 3 were assumed to be equal. Actually, however, it is both practical and necessary to have the motor 3 revolve faster than the motor 2 and to have the latter revolve faster than the motor I. Under such conditions, the shunt field 24, (Figs. 5 and 6) of the motor! will be made weaker than the shunt field 24 of the motor 2, and the shunt field of the latter will be made weaker than the shunt field 24 of the motor I. The result of this arrangement is that the revolving masses associated with the motor 2 are accelerated more slowly than those of the motor I while those of the motor 2 are accelerated still more slowly.

During the accelerating cycle all factors such as WR, R. P. M., and motor field strength bear a definite relation to the tension in the strip under actual rolling conditions. For difierent rolling schedules such as changes in width, draft, speed, and the like, new conditions are encountered. g

It is the main object of the invention to provide meansto introduce such flexibility into the rolling operations as will permit a predetermined setting of the shape of the motor speed curve characteristic. It may be desirable for certain rolling operations to run No. 3 mill stands at practically constant speed at all loading condimotor.

tions. It may also become necessary to run No. I mill stand at a drooping motor speed curve characteristic, or it may be necessary to select different settings of the motor speed curve charasteristic.

The invention, in one aspect, provides means for a predetermined setting of any desired mill motor speed curve characteristic. Reference is" now had to Fig. 5. The mill motor armature 21 is fed from the power buses 2|! and 2| The armature current of the armature 21 also passes through the field coil 22 of the motor driven series exciter generator. The armature 23 of the series excited generator furnishes the current for the series field coil 28 of the mfll The series exciter generator armature current also passes through the adjusting rheostats 29 and 20 whose function it is to provide means to raise or to lower the current through the series field coil 28 of the mill motor. Contactors 3|, 2|, 32-and 22' provide means to reverse the current in the series field coil 28, thus providing a bucking or boosting fiux to the flux developed by shunt field coil 24 of the mill motor. The shunt field coil 24 is fed from a separate excitation source 26 through an adjusting rheostat 25. If for an exemplary illustration, the currents through the series exciter generator field coils 22 of the mill motors l, 2, and 3 are equal, or of any desired ratio, the adjusting rheostats 29 and 30 provide means to raise or to erating principle of the devices shown in Fig. 5.

If the external load of the mill motor increases, more current will fiow through the motor annature. This higher armature current also flows through the field winding of the series exciter thereby increasing the voltage on the series exciter armature. This higher series exciter armature voltage will force more current through the series field winding of the mill motor. This higher field current changes the field fiux of the mill motor, which change can either be additive or subtractive to the existing shunt field flux depending on the presetting oi the contacts 3|, 3|, 32 and 32'. The adjustable rheostats in the series exciter armature circuit permit any desired current adjustment resulting in increased or decreased series field flux of the mill motor. The speed characteristics of the mill motors are therefore under full control permitting the setting of any desired shape of the speed curve thereof.

In another aspect of the invention means are provided as shown in Fig. 6 to'accomplish the desired changes in the motor speed curve characteristics'of the-motors 2, and 3.

The mill motor armature 21 is fed from the power busses 20 and 2| The armature current of the mill motor armature 2'! also passes through the armature 34, of the motor driven (not shown) booster or bucking generator 34. This armature current also passes through a shunt, or resistor 33. Changes in the voltage drop of the shunt or resistor 33 caused by currents of different amplitude passing therethrough, actuate the field regulators 36 which are of a well known type; these in turn raise or lower the current through the field coil 35 of the booster or bucking generator. accomplishes the same result as the arrangement of Fig. 5. The booster or bucking generator permits a change of the impressed armature voltage of the mill motor resulting in corresponding changes in motor speed. The change of the booster or bucking generator voltage is accomplished by the aid of the regulator in the field of the generator. The regulator is actuated by changes in voltage drop in the shunt or seriesresistor located in the armature circuit of, the

'mill motor. Thus a potential can be provided which is either additive or subtractive to the power bus potential 20 and 2|. A higher potential will force more current through the mill motor armature 21 thereby speeding up the This arrangement mill motor, a lower potential will lower the current through the mill motor armature 21 thereby lowering the speed of the mill motor. Thus, the invention again provides means to predeterminately adjust the speed curve characteristics of the mill motor.

Acceleration and deceleration, respectively, are accomplished by increasing and decreasing the gftential across the power buses 20 and Having thus described our invention, what we claim as new and desire to secure by Letters Patent, is:

1. The method of operating a tandem train oi. hot or cold mills, which includes providing motors therefor having adjustable speed-load characteristics, and adjusting the characteristics of said motors so that their curves are progressively different from the first to the last mill.

2. The method of reducing the thickness of a strip of metal in a tandem train of hot or cold mills, which consists in adjusting the speedload characteristic oi the first mill motor so as to be nearly flat, passing the strip through the first mill, adjusting the speed-load characteristic of the next mill motor so as to be of a different character and passing the strip therethrough, and

, continuing in this manner progressively until the strip is reduced to the desired gauge.

3. The method of operating a tandem train of hot or cold mills, which consists in providing eleclast mill.

4. Apparatus for rolling metal, comprising a tandem train of hot or cold mills, electric motors for said mills, said motors having means whereby their speed-load characteristics may be adjusted, and said motors being so adjusted that their speed-load characteristics are progressively different from the first to the last mill.

5. A tandem train of rolling mills having individual mill motors, and means in connection with said-mill motors to individually adjust their speedload characteristic curves to any predetermined shape.

6. A tandem train of rolling mills having individual mill motors, and means in connection with said mill motors to individually adjust their speed-load characteristic curves to any desired shape while a rolling operation is in progress.

EDWARD N. MILLAN. ERNEST G. .SCHLUP. 

